My investigation of reproductive behavior in male hamsters focuses on the interactions between environmental cues and steroid hormones in regulating the development and adult expression of reproductive and socially relevant communicative behaviors. In male hamsters, the components of mating behavior that are required for successful impregnation of females are critically dependent on both testicular hormones and female odor cues. Chemosensory information from females is transduced by the olfactory and vomeronasal system of males, and these systems convey essential neural input to the hypothalamus via specific pathways which contain androgen-concentrating neurons. (1) A primary objective of my research, using brain implant techniques, is to determine whether (a) testosterone action in these nuclei facilitates reproductive behavior and the emission of ultrasonic vocalizations, and if so, whether (b) these hormonal effects are dependent on, or are independent of, interactions with chemosensory stimulation. (2) A second objective is to determine whether perinatal testicular secretions are important for the development of (a) the male's attraction to sexually relevant female odors, and (b) the sensitivity of this response to testosterone facilitation in adulthood. Additional studies are designed (c) to learn how this attractant response may relate to specific chemoinvestigatory behaviors employed by the male when interacting with a receptive female, and (d) to discover which of these behaviors is important for acquiring the chemosensory stimulation that is necessary to arouse copulation. (3) A third objective is to investigate photoperiodic cues which are critical in regulating male hamsters' reproductive activity. Specifically, I will test the hypothesis that altered photoperiodic stimulation can suppress masculine copulatory behavior by its influence both on testosterone-dependent and testosterone-independent chemosensory mechanisms. Because of the critical effects exerted by a variety of internal and external cues in controlling male hamster sexual behavior, this species represents an ideal model system for investigating the neural and endocrine regulation of masculine reproductive behavior in mammals. The results from my studies will provide new and important advances in the fields of physiological psychology and behavioral endocrinology and neuroscience.